As shown in FIG. 1, a read/write (hereinafter referred to as R/W) circuit in a magnetic recording apparatus generally includes a R/W amplifier 12 connected to a head 10, an automatic gain control circuit (AGC) 14 connected to the differential outputs of the amplifier 12 through coupling capacitors C1 and C2, and a detector 16 for separately extracting data and servo signals from the output of the AGC 14. For reading, a signal read by the head 10 is amplified by the R/W amplifier 12 and then provided to the detector 16 through the capacitors C1 and C2, and the AGC 14. For writing, a write current is provided to the head 10 through a write amplifier (not shown) connected in parallel with the R/W amplifier 12. As is well known in the art, a servo signal from the detector 16 is used for positioning the head 10, and a data signal is provided to a digital processing circuit (not shown) such as a microprocessor.
In the above R/W circuit, it is known that there is a problem of DC offset at the output of the amplifier 12. That is, since DC levels at the output of the amplifier 12 on reading and writing differ from each other, a little time (this is called recovery time) is required for starting normal reading when the operation is switched from writing to reading. Various solutions have been proposed to reduce the recovery time so as to realize shorter access time and higher recording density in the magnetic recording apparatus, especially high speed hard disk drive (HDD).
For example, IBM Technical Disclosure Bulletin, Vol. 26, No. 4, Sep. 1983, pp. 2100 to 2103 describes a DC offset correction in which a fast DC restore loop including an integrator and two comparators is connected to one of the differential outputs of an AGC (corresponding to element 14 in FIG. 1), and the loop senses a new offset at the time of switching from writing to reading and rapidly adjusts an offset voltage at the time of previous writing. Further, Japanese Published Unexamined Patent Application 61-63966 discloses a circuit for shortening the recovery time in which the differential inputs of an AGC are short-circuited for a certain period of time at the time of switching from writing to reading.
As described above, the problem of delay in the recovery time due to a DC offset has been solved in some degree and a R/W amplifier such as HA166134LT manufactured by Hitachi, Ltd., which does not practically present a DC offset problem, has also been developed. However, there is a different problem other than the DC offset in such a R/W amplifier for magnetic recording. The R/W amplifier has a limited transition time when the operation is switched from writing to reading during which its output is quite indefinite and may vary widely depending on the components of the amplifier. Such indefinite output charges the coupling capacitors C1 and C2 shown in FIG. 1 to thereby shift a read output after the transition time, as in the case of the DC offset. Since the amount of such shift cannot be predicted, it has been assumed to have a possible maximum value in consideration of the maximum variation in the amplifier.
FIG. 2 shows output waveforms of the R/W amplifier 12, the AGC 14, and the detector 16 at the time of switching from writing to reading. Since the amplitudes of the R/W amplifier output and the AGC output are small as compared with those of the detector output, the R/W amplifier output and the AGC output are scaled up approximately a hundred times and twenty five times as large as the detector output, respectively in FIG. 2 (and FIG. 6 described later). A R/W control signal indicates writing when it is high and reading when it is low. During a transition time T after the R/W control signal is switched from high to low, the output of the R/W amplifier 12 largely swings (a part of its waveform is omitted by dotted lines) and its peak value is ten or more times as large as a peak value of the waveform after the transition time. It is seen that this large and irregular swing causes a DC level shift in the AGC output immediately after the transition time (in the example of FIG. 2, it is shifted upward) so that the detector output is delayed by a time Td. The figure shows reading of a servo area positioned at the beginning of each sector in a magnetic recording apparatus of a sector servo type. If the detector output is delayed in such an apparatus, a sector ID recorded at the beginning of the servo area may not be exactly detected. There would be no problem in the detection if the ID could be recorded taking account of the delay which, however, reduces the recording density per track. Since magnetic recording apparatuses having larger capacities are required, it is not preferable to have the detector output delayed. Particularly, in the magnetic recording apparatus of the sector servo type, the servo area is read from each sector even in a write operation so that reading and writing are alternately repeated for each sector, which would be affected by the delay of the detector output more strongly than the magnetic recording apparatus of the dedicated servo type.
Accordingly, it is an object of the present invention to provide a R/W circuit for a magnetic recording apparatus in which the output of an AGC is not shifted even if the output of a R/W amplifier irregularly varies during a transition time so that the output of a detector is not delayed.